This is about a different species so you can take it with a grain of salt.

I will often stop off at a pond on my way home from work and fish for sunfish. The visibility in the pond is pretty good and the fish are close to shore. On those occassions that I use a wet fly or streamer I can watch the following sequence.

1. The fish will become aware of the fly and swim over to it.
2. The fish will look at the fly for 20 - 30 seconds, then start to turn away.
3. I give the fly a 6-12" strip and stop.
4. The fish pounces on the fly.

I've also observed the same behavior with large & smallmouth bass.

I think that the stop-pause-start motion lets the fish think that the meal he thought was there and lost track of, is there, and excites the response of "your not getting away from me again"